AS MANY AS half a million Salvadorans may now be illegally in the United States, making this country a major sanctuary for victims of war and poverty in El Salvador. At the 1982 rate of deportations, moreover, 99 percent of that estimated total can expect to stay indefinitely. The flabbiness of the immigration law and the inability of the immigration service to apprehend or process more than a small fraction of the illegals make it so. A sharp argument has arisen, nonetheless, over that deported 1 percent. Are some in that number, denied political asylum, being deported to great peril?

Some church and human rights groups say they are, citing the pervasive violence in El Salvador and a few cases in which deportees have been killed. That the violence is so pervasive indicates that deportees may not be special targets of it. At the same time, it is unfeeling of the bureaucracy to suggest that the violence facing a given deportee doesn't count because it is pervasive rather than personal. Last year, 74 Salvadorans received political asylum. Of the 1,067 Salvadorans whose applications were denied, some simply had lawyers who know that requesting asylum is a good way to spin out an illegal stay, but conceivably some were worthy. Certainly the applicants should get the benefit of the inevitable doubt.

Especially for the churches now providing "sanctuary" for illegals, however, the point appears to be not simply to help people in trouble but in addition to use them in the cause of ending American aid to El Salvador. They would like all Salvadoran illegals to be treated as political refugees fleeing persecution, and they ask the administration to suspend the customary one-at-a-time immigration reviews and grant a blanket "extended voluntary departure" status permitting a mass indefinite stay.

Properly, we think, the administration resists spreading this blanket. The illegals are, after all, illegals. Their numbers are huge. Most are fleeing not violence or political persecution in El Salvador but economic hardship in Mexico, their port of first asylum. It diminishes the concept of political asylum to bestow that status unselectively. The better course is to treat, individually and compassionately, the relative handful of Salvadoran illegals who come into the coils of American law. Beyond that, the drive to rewrite the immigration law, so as to improve the American people's capacity to control who comes and goes, must move on.